1. Field of the Invention
The present invention relates to apparatus for protecting electrical equipment from otherwise-harmful surges of the type that may arise, for example, during electrical storms. More particularly, this invention pertains to a breaker mechanism for reliably removing a surge protector from operation at the end of its useful life.
2. Description of the Prior Art
Surge arrestors are commonly employed to protect domestic electric devices, particularly on overhead supply lines leading to individual buildings, from transient or persistent overvoltages. Arrestor devices are also employed in houses supplied by underground cable as voltage surges can also occur in supply systems limited to underground cables just as in those using a combination of underground cables and overhead lines. Such devices are additionally employed to protect individual electrical devices that are particularly sensitive to overvoltages.
Surge arrestors act to limit the intensity of transient or persistent overvoltages. Low voltage arrestors employed in office buildings that have electrical installations, hospitals, lighting towers and installations protected by earth leakage circuit breakers prevent the undesired switching of such breakers which could otherwise be triggered by substantial overvoltages.
Surge or lightning arrestors typically employ voltage-dependent resistors, such as a silicon carbide or metal oxide (MOV) varistors. Such devices act somewhat in the manner of a Zener diode, with resistance to current flow decreasing as voltage increases to limit the maximum potential difference. At very high overvoltages, large current flow through the varistor circuit bleed the voltage peak levels on the line conductor to protect the connected electrical devices, with the varistor (or a parallel combination thereof) thereby acting as a voltage clamp.
Over time, such large flows of current will physically degrade the varistor. Each time a surge occurs, some damage will take place, increasing the leakage current of the device relative to a given voltage change. After an accumulation of damage over time, excessive device leakage currents will generate enough i.sup.2 R heating to burn a hole in the device. The "end of life" of a lightning arrestor device means it will short circuit and draw excessive current until an interruption takes place or the device explodes, burns wires and causes fires.
Catastrophic failure of a surge arrestor can be quite dangerous, producing both fires and explosions. Accordingly, attempts have been made to remove or disconnect the surge arrestor from the line conductor before the end of its useful life. Generally, such efforts have relied upon the insertion of a fuse in series with the varistor circuit. Such a fuse will "blow" to disconnect the device from the a.c. or d.c. circuit by sensing the amount of current being drawn.
A significant drawback of such arrangements has been the difficulty of coordinating the action of the fuse with varistor failure. During certain events, such as thunderstorms, very large voltage transients may appear upon the line conductor. Such transients can induce large current flows that are of sufficient strength to cause the fuse to blow. However, the surge capacity of a fuse is not a direct measure of the integrity of the varistor's physical status which, as mentioned, is a function of time in the sense that damage to its current-handling capacity is cumulative. Thus, a fuse might blow despite the varistor's ability to handle many more episodes of overvoltage. In the case of a thunderstorm, which may last many hours, electrical devices can be left unprotected against overvoltage damage for hours by a fuse's blowing as a result of a current surge occurring relatively early. Thus fuse arrangements face the inherent design contradiction posed by the need for assured activation by a low resistance short while attaining sufficient current handling capacity to prevent premature surge arrestor disconnection.